Welding of cylindrical structures is well known and the particular materials and uses of the welded materials will often dictate the manner of welding. For example, where high-pressure vessels or pipes are welded, manual or partially automated arc welding processes (e.g., GMAW, GTAW, SMAW, etc.) are often employed. Alternatively, and especially for pipes with moderate diameters, fully automated orbital arc welding is also known. However, fully automated orbital arc welding is often limited to specific welding methods (commonly TIG) and requires significant user experience. Moreover, arc welding is often not suitable or desirable for rapid joining of relatively thin metal structures.
In still another known welding method, friction stir welding, two materials are joined together in a solid state process where a typically non-consumable tool is rotated in the joint line between two materials under conditions to soften but not melt the two materials. As the tool rotates and advances along the joint line, the softened materials are stirred together to form a highly stable weld joint. Friction stir welding is especially advantageous for pipes that can not be subjected to post weld heat treatment methods, and/or pipes that have stringent quality requirements. Still further, friction stir welding requires significant forces and is thus often not suitable for field use. Moreover, due to the significant forces, relatively thin metal structures will typically deform under the severe process conditions.
Relatively thin metal structures are often joined by resistance welding, which is based on the temporary melting of surfaces of contacting materials, wherein the heat is generated by the electrical resistance against very high currents (e.g., several thousand Amp) at the point of contact. Consequently, critical parameters for the welding process will typically include the type of material and proportions of the work pieces, the electrode materials and electrode geometry, electrode pressing force, the weld current, and weld time, etc. While resistance welding is relatively simple for flat and thin work pieces, resistance welding of curved materials to curved, and especially tubular materials is often challenging. Such problems are further compounded where the curved materials require continuous weld seams rather than weld spots.
In certain cases, resistance welding can be performed using wheel-shaped electrodes as can be seen from U.S. Pat. Nos. 6,323,453 or 6,281,467. However, such an electrode assembly is generally limited to resistance welding of flat objects. In other cases, resistance welds are placed on cylindrical objects in longitudinal direction using wheel-shaped electrodes as described in EP 0273607 or GB 1136736, and cylindrical objects can be welded to flat objects as shown in U.S. Pat. No. 2,809,276. While such configurations and methods advantageously simplify the welding process, they are typically not suitable to join two cylindrical structures end-to-end. In still other known methods, resistance welding is used to produce a circumferential weld on a can using two wheel-shaped electrodes that are statically positioned onto opposite sides of the cylindrical object as described in U.S. Pat. No. 4,661,673. While such arrangement allows circumferential welding, various disadvantages nevertheless remain. For example, due to the placement and operation of the electrodes, two welds are simultaneously created that are 180° apart, which may lead to misalignment of the welds. Moreover, such weld devices are once again generally not suitable for joining tubular structures end-to-end or joining tubular structures to thin metal sheets.
Thus, even though numerous methods and configurations for resistance welding are known in the art, there is still a need for welding device and methods that allow welding of curved materials, and especially welding of curved materials to tubular structures.